The present invention relates to a system for driving and controlling the speed of permanent-magnet direct-current motors with a rotor winding comprising a coil without an iron core and of low inertia. These motors are used for the drive of the magnetic tape in equipment for recording and/or playing back magnetic tape cartridges and cassettes.
A motor of this type is fully described in the booklet entitled "Kleine Gleichstrommotoren mit eisenfreien Anker" (Small direct-current motors with iron-free armatures) by Dr. Otto Stemme, published by Interelectric A.G., Sachseln, Switzerland. Another motor suitable for the purposes of the present invention is that described in U.S. Pat. No. 3,467,847.
These motors are characterized by a very low inertia of the rotor because it is formed solely by the winding, completely lacking a rotating iron core; moreover, owing to their construction, they have a rotor inductance which is very low and, therefore, negligible with respect to the other electric quantities in action, so that it can be considered with good approximation that the speed of rotation of the motor is a linear function of the current supplied to it. Therefore, by varying the current supplied to the motor in any known manner whatsoever, the result will be obtained that the speed of rotation of the motor itself will follow the course taken by the current.
The use of magnetic tape cassettes of the type running at 12.5 inches per second (31.75 cm per second) is spreading more and more in the field of data transmission and in particular in electronic machines such as data collecting systems, small calculators, cash registers, etc. Because of the high operating speed of the electronic machines, optimization of the use of the magnetic tape is necessary. This is achieved by controlling and minimizing the running times of the tape also during the stages of starting up and braking the motor driving the tape. Moreover, a very uniform normal-working driving speed is required, with variations contained within predetermined limits.
It is known to use a reference voltage generating circuit connected to control a current supply circuit and adapted to generate a rising voltage slope for the starting function of the motor, a falling voltage slope for the braking function of the motor, and a constant voltage during the period between the two slopes for the normal-speed operation of the motor.
Apparatus used for driving the magnetic tape of cassettes is known in which the regulation of the speed is effected solely during the normal operation of the motor. In other arrangements known for reducing the braking time of the motor there are employed mechanical friction brakes of bulky and costly construction and the operation of which is limited by the wear of the rubbing elements. The objects mentioned above are achieved by the system described in the present specification, which enables not more than 2,9 cm of tape to run past the playback and/or recording head during the starting and braking stages, and is capable of maintaining variations of speed within 5% of the nominal value under the worst operating conditions.